1. Field of Invention
The invention relates to a separator for a low temperature type fuel cell, especially, a polymer electrolyte fuel cell (hereinafter referred to as PEFC), and more particularly, to a metal plate of the separator, to which different types of surface treatment is applied.
2. Description of Related Art
The PEFC is formed by stacking an MEA (Membrane-Electrode Assembly) and separators. The MEA includes an electrode or anode (fuel electrode) formed of an electrolyte membrane as an ion-exchange membrane and a catalytic layer formed on one surface of the electrolyte membrane, and an electrode or cathode (air electrode) formed of the electrolyte membrane as the ion-exchange membrane and a catalytic layer formed on the other surface of the electrolyte membrane. Diffusion layers are provided between the MEA and the separators at the anode side and the cathode side, respectively. The separator has a fuel gas passage for supplying the fuel gas (hydrogen) to the anode, and has an oxide gas passage for supplying oxide gas to the cathode. The separator also has a refrigerant passage through which a refrigerant or cooling water is supplied. A unit cell is formed by interposing the MEA between the separators. At least one unit cell is used to form a module, and a plurality of modules are further stacked into a fuel cell stack. Terminals, insulators and end plates are provided at both sides of the fuel cell stack in the stack direction such that the cells are tightened in the stack direction so as to be fixed with fastening members, for example, a tension plate that is disposed outside the fuel cell stack and extends in the stack direction, and bolt/nut into the fuel cell stack structure. At the anode side of each cell, a reaction occurs for decomposing hydrogen into a hydrogen ion (proton) and an electron. The resultant hydrogen ion moves toward the cathode side through the electrolyte membrane. At the cathode side of each cell, the hydrogen ion and the electron (generated in the anode of the adjacent MEA through the separator, or generated in the anode of the cell at one end in the stack direction moving to the cathode of the cell at the other end through the outer circuit) are reacted to generate water as follows:                anode: H2→2H++2e−        cathode: 2H++2e−+(½)O2→H2O.The cell voltage by each cell or a group of cells is monitored so as to make sure if power is normally generated in the cell, to control the flow rate of reaction gas, and to guide the motor in case of abnormal voltage. For example, JP-A-11-389828 discloses a cell voltage monitor for the fuel cell. JP-A-2001-283880 discloses application of carbon coat to a whole surface of the metal separator so as to improve the corrosion resistance of the gas passage portion thereof.        
If the carbon coat is applied to a contact portion of the metal separator, which is brought into contact with a terminal of a cell voltage monitor, the contact resistance of such contact portion becomes unstable. As a result, the accuracy in detecting the voltage generated in the cell is degraded. If the carbon coat is not applied to the metal separator, the corrosion in the gas passage portion may rapidly progress. It is difficult for the aforementioned technology to stabilize the contact resistance of the contact portion while improving the corrosion resistance of the gas passage.